drill holes at specified offsets
All sorts of ways, making your own measuring sticks, temporary edges or loose vertices to snap to, etc, etc, but here's an example for putting holes in a tube at measured Z positions using the cursor...


Make a section with a hole, and in edit mode with all selected,
ShiftS > 'Cursor to Selected':





ShiftD duplicate the section.
In the Properties Region, 3D Cursor panel, set the Cursor to the desired Z
The duplicate section should be selected...  ShiftS again > 'Selection to Cursor (Offset)





Rinse, repeat ...
CtrlE > 'Bridge Edge Loops' to fill the gaps




If you mean from other edges, you can place the cursor at the median of any selection of elements in Edit Mode, and you can enter arithmetic expressions in its location fields.. so if the cursor was at, say, 1.414, you can just add a "-0.5" to the end of the existing value to get your offset.